V23849-R3x-C55

Application Notes

Preliminary Product Information

19

2004-06-28

Application Notes

EMI Recommendations

To avoid electromagnetic radiation exceeding the required limits set by the standards,

please take note of the following recommendations.

When Gigabit switching components are found on a PCB (e.g. multiplexer,

serializer-deserializer, clock data recovery, etc.), any opening of the chassis may leak

radiation; this may also occur at chassis slots other than that of the device itself. Thus

every mechanical opening or aperture should be as small as feasible and its length

carefully considered.

On the board itself, every data connection should be an impedance matched line (e.g.

strip line or coplanar strip line). Data (D) and Data-not (Dn) should be routed

symmetrically. Vias should be avoided. Where internal termination inside an IC or a

transceiver is not present, a line terminating resistor must be provided. The decision of

how best to establish a ground depends on many boundary conditions. This decision

may turn out to be critical for achieving lowest EMI performance. At RF frequencies the

ground plane will always carry some amount of RF noise. Thus the ground and

V

CC

planes are often major radiators inside an enclosure. As a general rule, for small systems

such as PCI cards placed inside poorly shielded enclosures, the common ground

scheme has often proven to be most effective in reducing RF emissions. In a common

ground scheme, the PCI card becomes more equipotential with the chassis ground. As

a result, the overall radiation will decrease. In a common ground scheme, it is strongly

recommended to provide a proper contact between signal ground and chassis ground at

every location where possible. This concept is designed to avoid hotspots which are

places of highest radiation, caused when only a few connections between chassis and

signal grounds exist. Compensation currents would concentrate at these connections,

causing radiation. However, as signal ground may be the main cause for parasitic

radiation, connecting chassis ground and signal ground at the wrong place may result in

enhanced RF emissions.

For

example,

connecting

chassis

ground

and

signal

ground

at

a

front

panel/bezel/chassis by means of a fiber optic transceiver/cage may result in a large

amount of radiation especially where combined with an inadequate number of grounding

points between signal ground and chassis ground. Thus the transceiver becomes a

single contact point increasing radiation emissions. Even a capacitive coupling between

signal ground and chassis ground may be harmful if it is too close to an opening or an

aperture. For a number of systems, enforcing a strict separation of signal ground from

chassis ground may be advantageous, providing the housing does not present any slots

or other discontinuities. This separate ground concept seems to be more suitable in large

systems where appropriate shielding measures have also been implemented.